1. Field of the Invention
The present invention relates to novel aqueous silicone emulsions crosslinkable into elastomeric state upon removal of water therefrom.
2. Description of the Prior Art
U.S. Pat. No. 2,891,920 describes a process for the emulsion polymerization of polydiorganosiloxanes employing an acidic or basic catalyst in the presence of anionic, cationic or nonionic surface-active agents. This patent notes that the emulsions produced are stable in storage and, after the addition of fillers, can be used to formulate paints which form continuous coatings on removal of water.
U.S. Pat. No. 3,294,725 describes, in particular, the use of dodecylbenzenesulfonic acid to polymerize polydiorganosiloxanes in emulsion. The '725 patent teaches that, in order to produce stable emulsions, it is desirable to adjust the pH of the emulsions to a value of approximately 7. This patent also notes that an elastomeric coating can be obtained from such neutralized emulsions, to which colloidal silica and a polyalkoxysilane have been added.
The disclosure of U.S. Pat. No. 3,360,491 is similar to that of U.S. Pat. No. 3,294,725, except that dodecylbenzenesulfonic acid is replaced with lauryl hydrogen sulfate.
U.S. Pat. No. 3,355,406 describes, inter alia, a silicone latex comprising:
(i) an aqueous colloidal suspension of an .alpha.,.omega.-(dihydroxy)polydiorganosiloxane;
(ii) a filler necessarily comprising a silsesquioxane in the form of a colloidal suspension; and
(iii) a catalyst selected from among inorganic acids, strong bases, dialkyltin diacylates and organic or inorganic peroxides.
U.S. Pat. No. 3,697,469 describes a particular process for emulsion polymerization of polydiorganosiloxanes and indicates the possibility of adding colloidal silica and a tin salt to such emulsion with a view to providing an elastomeric coating by evaporation of water.
French Patent FR-A-2,110,358 describes a silicone emulsion at a pH of from 6.5 to 9, which crosslinks into an electrically conducting elastomer after evaporation of water on the incorporation of carbon black. The emulsion, which also contains a tin salt and a polyalkoxysilane, is not stable in storage and must be stored in two separate containers (two-component emulsion).
U.S. Pat. Nos. 4,221,688 and 4,244,849, and French Patent FR-A-2,463,163, describe silicone emulsions which are stable in storage and which comprise:
(i) an anionically stabilized emulsion of an .alpha.,.omega.- (dihydroxy)polydiorganosiloxane polymer;
(ii) a siliceous filler;
(iii) a tin salt; and
(iv) optionally, a nonreinforcing filler.
The siliceous filler may be a colloidal silica (U.S. Pat. No. 4,221,688), sodium silicate (U.S. Pat. No. 4,224,849) or an amorphous silica powder (FR-A-2,463,163).
When compared to the known aqueous emulsions of the prior art, these three patents teach, on the one hand, that in order to produce a single-component emulsion which is stable in storage, it is necessary to maintain the emulsion at an alkaline pH above 8.5 or 9, preferably above 10 and, on the other hand, to incorporate a tin salt in the emulsion in order to shorten to a few days the emulsion aging stage necessary to provide an acceptable storage stability before it is packaged in a single container.
While the adjustment of the emulsion to an alkaline pH has made it possible to improve the storage stability in comparison with the known emulsions stored at a neutral or basic pH, it has become apparent that the elastomer obtained after water evaporation exhibits a major change in its mechanical properties over time, in particular in respect of elongation, which may be disadvantageous.
This change in mechanical properties is not a surprising phenomenon in this art. Indeed, it is well known that, in an alkaline medium, silicone polymers depolymerize to low molecular weight species, including the monomer species.
Such scissions of the silicone chains are, of course, the major factor in the changes in mechanical properties of the elastomer over time, namely, during its aging.
Furthermore, known filled silicone emulsions stored at an acidic or neutral pH exhibit at least one of the following disadvantages:
(a) they are not stable in storage when they are packaged in a single container (single-component composition);
(b) they exhibit an excessively long time for curing into an elastomer; and
(c) they are not thixotropic.